1. Field of the Invention
The present invention relates generally to an apparatus and method for detecting a Cyclic Prefix (CP) length in a mobile communication system. More particularly, the present invention relates to an apparatus and method for detecting a CP length in a Long-Term Evolution (LTE) communication system terminal.
2. Description of the Related Art
A Long-Term Evolution (LTE) communication system uses Orthogonal Frequency Division Multiplexing (OFDM) in a downlink. Therefore, it is necessary to add a Cyclic Prefix (CP) to each data symbol in the downlink in order to prevent Inter-Symbol Interference (ISI). Furthermore, depending on channel delay spreads, the CP may have either a normal length or an extended length.
The use of Fast Fourier Transform (FFT) is necessary to demodulate the data symbol in a system using OFDM. Accordingly, it is essential to know the length of the CP so that it can be discarded or otherwise ignored as part of the FFT process, leaving only the data for demodulation. Also, knowing the CP length is essential for initial terminal synchronization.
Specifically, knowing the CP length is essential in detecting a Secondary-Synchronization CHannel (S-SCH) in the LTE communication system. Since the S-SCH is located in a symbol directly before an OFDM symbol for transmission of a Primary-Synchronization CHannel (P-SCH), it is not possible to know whether the CP length is the normal type or the extended type.
In general, a correlation operation is performed in a frequency domain in order to detect the S-SCH. In this case, foreknowing the CP length is essential for FFT of an OFDM symbol.
FIG. 1 is a block diagram illustrating a conventional CP length detector.
FIG. 1 illustrates an apparatus for calculating an average value of CP correlation. For calculation of the CP correlation average value, it is understood that 7 OFDM symbols are contained in each slot (i.e. 64 samples=10×1+9×6) when a normal CP is used and 6 OFDM symbols are contained in each slot (i.e. 192 samples=32×6) when an extended CP is used.
In consideration of a frame structure for each case, an enable control signal is used to determine whether to move a calculation interval for a CP correlation average value.
A 64-point moving average unit 130 calculates a CP correlation average value for a 64-point interval, and a 192-point moving average unit 140 calculates a CP correlation average value for a 192-point interval.
A normalization/determination unit 150 determines and detects a CP length on the basis of the CP correlation average value calculated by the 64-point moving average unit 130 and the 192-point moving average unit 140.
Referring to FIG. 1, a memory size is estimated on the assumption that a 5-ms half frame boundary is known by P-SCH detection.
That is, on the assumption that the 5-ms half frame boundary is known, an accurate slot boundary in a serving cell is calculated and a CP length is determined accordingly.
Whether a CP is a normal type or an extended type is determined according to the determined CP length.
In this case, a necessary memory size is about 3072 bits (i.e. 64+192 samples×12 bits/sample). Such a memory size may vary slightly depending on the number of valid bits of a cumulative value resulting from the correlation. Herein, it is set to 12 bits.
However, there may be a case in which the 5-ms half frame boundary is unknown, i.e., the case that the CP length is detected simultaneously with the P-SCH detection.
In this case, a search time can be reduced. However, all the information from 1 slot (e.g., 960 samples) must be stored because there is no information about the 5-ms half frame boundary. In this case, an estimated memory size is about 11520 bits (i.e. 960 samples×12 bits/sample), which is about four times the normal case.
Also, in this case, there is a higher probability that an extended CP is misinterpreted as a normal CP.